Lunar Age Calculator

Date

Time (HH:MM)

Optional. Noon is used if empty.

Timezone offset (hours)

Example: +5, 0, -4.5

Decimal places

Synodic month (days)

Reference new moon (JD)

Calculate Lunar Age

Date	Time	TZ Offset	Lunar Age (days)	Phase
2026-03-13	12:00	0	24.52	Waning Crescent
2025-01-01	00:00	0	1.45	New Moon
2026-01-01	18:00	+5	12.62	Waxing Gibbous
2030-06-15	06:30	0	14.17	Full Moon

This calculator estimates lunar age as the time since the most recent new moon. It converts your date and time to a Julian Day (JD) in UTC. Then it computes the remainder of the elapsed days within one synodic month.

1) Julian Day

A standard Gregorian-to-JD conversion is used, including a fractional day from the clock time.

2) Lunar age (days since new moon)

Age = (JD − JD0) mod S
where S = 29.530588853 days (mean synodic month) and JD0 = 2451550.09765 is a reference new moon Julian Day.

3) Phase angle and illumination (approx.)

PhaseAngle = 360 × Age / S
Illumination = (1 − cos(PhaseAngle)) / 2
Illumination is reported as a percentage from 0% (new) to 100% (full).

This is an approximation. Local horizon, parallax, and orbital perturbations are not modeled.

1. Enter the calendar date you want to check.
2. Optionally enter a local time to refine the estimate.
3. Set your timezone offset in hours (example: +5).
4. Choose decimal places for rounding.
5. Press Calculate Lunar Age to view results above.
6. Use Download CSV or Download PDF for saving.

Tip: For quick planning, using noon and your offset is often sufficient.

1) What lunar age tells you

Lunar age is the number of days since the last new moon. It is a practical way to describe where the Moon sits in its repeating light cycle. When the age is near 0 days, the Moon is near new. Near 14.8 days, it is near full. This calculator reports age, a phase label, and a simple illumination estimate.

2) Synodic month data

The repeating new‑to‑new cycle is called the synodic month. Its mean length is 29.530588853 days. That value is used as S in the computation, so your date is placed onto a 0–S scale. Real months vary by several hours because the Moon’s orbit is not perfectly uniform.

3) Early cycle: new to first quarter

In the first week, the age grows from about 0 to about 7.4 days. The calculator labels this span as New Moon and Waxing Crescent, then reaches First Quarter near 9.2 days in the simple eight‑phase table. A young crescent is often best for sunsets and low‑contrast landscape scenes.

4) Mid cycle: first quarter to full

From roughly 9 to 14.8 days, the Moon is waxing gibbous. The illuminated fraction increases quickly, and the Moon rises earlier each night. Around the midpoint of the cycle (about 14.8 days), the Moon is near full and stays up most of the night, brightening the sky for deep‑sky observing.

5) Late cycle: full to last quarter

After full, the age continues toward 29.53 days and the phase becomes waning. The calculator switches through Waning Gibbous and reaches Last Quarter near 24.0 days in the same table. Late‑cycle nights can be excellent for morning observing because the Moon rises after midnight.

6) Illumination and phase angle

The phase angle is computed as 360 × Age / S. Illumination is then estimated with (1 − cos(angle))/2, giving 0% near new and 100% near full. In reality, libration and geometry shift the apparent fraction slightly, but this model is a useful planning number.

7) Planning with “days to next” values

Two countdowns are shown: days to the next new moon and days to the next full moon. These values are derived from your age within the synodic month, so they change smoothly as you adjust the time. Use them to pick darker nights for astronomy, or brighter nights for moonlit hikes and photo sessions.

8) Why your result may differ elsewhere

High‑precision ephemeris tools account for orbital perturbations, the observer’s location, and the exact definition of “phase moment.” This calculator uses a stable reference new moon and a mean month length, so differences of a few hours are normal. For eclipse work or navigation, use a dedicated ephemeris source.

1) What does “lunar age” mean?
It is the time, in days, since the most recent new moon. It helps you estimate whether the Moon is a crescent, quarter, gibbous, or full.

2) Why do I need a timezone offset?
The calculation uses UTC internally. The offset converts your local clock to UTC so the date and time map to the correct Julian Day.

3) How accurate is this calculator?
It is designed for planning and education. Expect results to be close, but not identical to high‑precision ephemerides. Differences of several hours can occur.

4) What is Julian Day?
Julian Day is a continuous day count used in astronomy. It avoids calendar complications and makes time intervals easy to compute.

5) What is the lunation number shown here?
It is an approximate cycle count from the reference new moon used in this tool. It is useful for grouping dates into repeating lunar months, not for official catalogs.

6) Can I use this for eclipses?
No. Eclipses depend on the Moon’s node alignment and exact geometry. Use an eclipse‑focused ephemeris or an astronomy almanac for reliable predictions.

7) How do the CSV and PDF downloads work?
After you calculate, the tool stores your latest result in the session. The download buttons export that same result so your report matches what you just viewed.